This invention relates to the fabrication of semiconductor devices, and more specifically to the fabrication of silicon-based (Si-based) light emitting diodes (LEDs) and laser diodes (LDs) using nano-patterning and direct wafer bonding techniques.
As the term is used herein, direct-wafer-bonding is intended to mean a process whereby two smooth and flat surfaces are brought together, in physical contact, in the absence of an intermediate layer or film, and usually with the application of a uniaxial pressure, such that the two flat surfaces are locally attracted to each other by Van der Walls forces, so that the two flat surfaces stick or bond together. The crystallites in the two flat surfaces of a direct-wafer-bonded interface can fuse together at elevated temperatures due to the surface-energy-induced migration and growth, or the formation of bonds, between the two surface species.
Silicon is a semiconductor of choice for integrated circuits and electronic devices. However, silicon has an indirect bandgap of about 1.1 eV, which makes silicon a relatively inefficient light emitter.
It has been predicted that reducing the physical size of a silicon crystal in all three dimensions, to that of a quantum box or a quantum dot, forces silicon to behave as a direct bandgap material, and therefore become suitable for optical purposes. Moreover, one can tailor the light emission wavelength throughout the visible spectrum by changing the physical dimensions of the silicon quantum dots (Si QDs). For example, see U.S. Pat. Nos. 5,559,822 and 5,703,896, incorporated herein by reference.
One utility of the present invention is to fabricate a double heterostructure (DH) laser diode (LD). A DH LD is described in U.S. Pat. No. 3,309,553, incorporated herein by reference.
The present invention makes use of nano-patterning. The use of bionanomasks as nanometer-scale patterning masks is described in U.S. Pat. No. 4,802,951, incorporated herein by reference.
It is believed that United States patent applications have been filed describing the creation of the nanodot masks, and a technique for tuning the diameter of the nanodots.
DH AlInGaP p-n diodes that are wafer bonded to GaP are typically used for red LEDs (see F. A. Kish et al. Appl. Phys. Lett. 64, 2839, 1994), while DH InGaN p-n diodes are used for green, blue and white LEDs (see S. Nakamura and G. Fasol, The blue laser diode, Springer, Berlin, 1997).
As described in the publication LONG-WAVELENGTH SEMICONDUCTOR LASERS (G. P. Agrawal and N. K. Dutta, ATandT Bell Laboratories Murray Hill, New Jersey, Van Nostrand Reinhold, New York) it has been suggested that semiconductor lasers might be improved if a layer of one semiconductor material were sandwiched between two cladding layers of another semiconductor material that has a relatively wider band gap. Such a device consisting of two dissimilar semiconductors is commonly referred to as a heterostructure laser, in contrast to single-semiconductor devices called homostructure lasers. Heterostructure lasers are further classified as single-heterostructure or double-heterostructure devices, depending on whether the active region, where lasing occurs, is surrounded on one or both sides by a cladding layer of higher band gap.
FIG. 1 provides an example of a prior art double-heterojunction semiconductor laser 10 having typical dimensions as shown, wherein the hatched area is a thin, about 0.2 micrometer thick, active layer 11 of a semiconductor material whose band gap is slightly lower than that of the two surrounding cladding layers 12 and 13.
This invention uses direct-wafer-bonding as a fabrication tool to make Si-based light emitters.
In accordance with this invention, a technique known as nano-patterning is used to fabricate, and to control the size of, an ordered array or an ordered matrix of Si QDs, and direct-wafer-bonding provides that a layer of the nano-patterned Si QDs is placed between, or integrated into, two closely adjacent silicon carbide (SiC) cladding/contact layers or wafers, thereby forming a double heterostructure p-n light emitting diode.
The emission wavelength of DH devices in accordance with the invention can be tuned from the infrared part of the spectrum into the ultraviolet part of the spectrum by changing the physical size of the Si QDs.
The invention provides a method of making direct-wafer-bonded, Si-based, DH light emitting diodes (LEDs), and direct-wafer-bonded, Si-based, DH LDs, by sandwiching a layer of Si QDs between two SiC cladding layers or wafers, one SiC cladding layer being an n-type cladding layer, and the other SiC layer being a p-type cladding layer.
SiC is a wide band gap semiconductor, having a band gap energy ranging from about 2.5 to about 3.2 eV, depending upon the polytype, and SiC has an index of refraction (about 2.63) that is smaller than that of Si (about 3.44). The two SiC cladding layers within DH devices in accordance with the invention therefore provide both electrical confinement and optical confinement inside of the Si QDs.
DH devices in accordance with the invention consist of three distinct semiconductor layers, respectively called (1) a wide band gap, n-type, cladding/contact layer, (2) a wide band gap, p-type, cladding/contact layer, (3) and a narrow band gap, nominally indirect band gap, active layer or region that lies between the two wide band gap cladding/contact layers.
More specifically, and in a non-limiting embodiment of the invention, the wide band gap n-type cladding/contact layer is n-SiC, the wide band gap p-type cladding/contact layer is p-SIC, and the narrow and indirect band gap active layer comprises a plurality of Si quantum dots, each quantum dot having a thickness that is no greater than about 250 Angstroms, each quantum dot having a width dimension that is no greater than about 200 Angstroms, and the quantum dots having a center-to-center spacing in the range of from about 10 Angstroms to about 1000 Angstroms. At less than a critical center-to-center spacing, electrons tend to tunnel between adjacent quantum dots.
Direct-wafer-bonded, Si-based, DH LEDs in accordance with this invention can be used for displays and for lighting purposes, while direct-wafer-bonded, Si-based, DH LDs in accordance with this invention are useful for communication, storage, printing purposes and photochemistry.